The polymer fine particles mean fine particles composed of a polymer, and the fine particles generally have a wide range of diameters from several tens of nm to several hundreds of μm. Different from polymer molded products such as films, fibers, injection molded products, and extrusion molded products, polymer fine particles are used for modification and improvement of various materials by utilizing the large specific surface area and the structure of fine particles. Their major uses include modifiers for cosmetics, additives for toners, rheology modifiers for paints and the like, agents for medical diagnosis and examination, and additives for molded products such as automobile materials and construction materials.
In particular, they have been in wider use in recent years due to the advantageous particle structure of polymer fine particles, as materials for rapid prototyping and rapid manufacturing, i.e., techniques to produce custom-made molded products combining with laser processing technologies. To yield the best possible effect of the fine particles in such a use, there are increasing demands for polymer fine particles having various properties such as high sphericity and narrow particle size distribution in addition to high mechanical properties, high heat resistance and high solvent resistance.
Under such a situation, polyamide fine particles are being developed in recent years from the viewpoint of their excellent fracture toughness, impact resistance and chemical resistance, and their good heat resistance and solvent resistance, and are used in a variety of areas such as modifiers for epoxy resin, cosmetic foundations, and materials for rapid prototyping.
However, due to the increasing variety of various materials, a situation arises where there are growing demands for properties and characteristics which cannot be achieved by conventional polyamide fine particles.
Composite particle solutions of polyamide fine particles in combination with a different material are proposed to deal with such a situation (see JP-A-2008-239638, JP-A-2010-18684 and JP-A-2010-132811).
However, many of these composite particle solutions are solutions in combination with an inorganic material which impair the intrinsic properties of polyamide fine particles such as excellent fracture toughness and flexibility. To maintain the intrinsic properties of polyamide fine particles and yield a further improvement in their properties and characteristics, it is preferred to produce a composite of a polyamide in combination with a different polymer. One example of such a composite polymer fine particle is a core-shell structure. However, when polyamide component forms shell, when the shell is thin the properties of core components are exhibited strongly instead of the intrinsic properties of polyamide fine particles, and when the shell is thick the effect of the composition becomes small. On the other hand, when polyamide component forms core, although there may be an advantage in surface modification of polyamide fine particles, there are inadequacies in improving the own properties and characteristics of fine particles. Due to these reasons and from the viewpoint of improvements in polymer properties and characteristics obtained by the composition, a preferred structure of composite polyamide fine particles is a structure in which a different component is dispersed in a matrix and whose representative example is an ABS resin.
As an example of the composition of polyamide fine particles, there are disclosed composite polyamide fine particles obtained by removing media components after melting and kneading all raw materials of polymers (see JP-B-4574978), and it is presumed that polymer fine particles having a sea-island structure or the like can be obtained when there is a high compatibility between polyamide and a polymer to be composited. However, when their difference is relatively large, namely, in the case where the compatibility between polyamide and a polymer to be composited is relatively low, only a core-shell structure is obtained. Thus, there is substantially no disclosure of a method of obtaining a desired dispersion structure.
Additionally, although the composite polymer fine particles obtained by that method are mostly spherical or globular in shape, the fine particles contain irregular ones partially, thus it is necessary to remove these irregular fine particles when globular particles are required. However, it is very difficult to remove only irregular ones from a huge number of particles.
Further, in that method it is necessary to raise the kneading temperature to the melting point of polymer material or higher. Thus, when the fusion temperatures of polymers are largely different from each other, it is difficult to control the composition because of the problems such as the viscosity difference between polymer materials or the like.
As described above, it is the current status that spherical composite polyamide fine particles and a method of producing thereof are not discovered yet, the spherical composite polyamide fine particles having a structure in which, in a polyamide matrix, a different polymer having a relatively low compatibility is dispersed.
It could therefore be helpful to provide novel composite polyamide fine particles in which excellent fracture toughness, impact resistance, chemical resistance and the like intrinsic to polyamide fine particles are not impaired and which have various properties and characteristics such as high fracture toughness, impact resistance and heat resistance that cannot be achieved by polymer fine particles comprising a single type of polyamide, and to provide a method of industrially and advantageously producing the composite polyamide fine particles without being limited by the types of polymer to be composited.